Combined Treatment of Xenon and Hypothermia in Newborn Rats - Additive or Synergistic Effect?

BACKGROUND: Breathing the inert gas Xenon (Xe) enhances hypothermic (HT) neuroprotection after hypoxia-ischemia (HI) in small and large newborn animal models. The underlying mechanism of the enhancement is not yet fully understood, but the combined effect of Xe and HT could either be synergistic (larger than the two effects added) or simply additive. A previously published study, using unilateral carotid ligation followed by hypoxia in seven day old (P7) rats, showed that the combination of mild HT (35°C) and low Xe concentration (20%), both not being neuroprotective alone, had a synergistic effect and was neuroprotective when both were started with a 4 h delay after a moderate HI insult. To examine whether another laboratory could confirm this finding, we repeated key aspects of the study. DESIGN/METHODS: After the HI-insult 120 pups were exposed to different post-insult treatments: three temperatures (normothermia (NT) NT(37°C), HT(35°C), HT(32°C)) or Xe concentrations (0%, 20% or 50%) starting either immediately or with a 4 h delay. To assess the synergistic potency of Xe-HT, a second set (n = 101) of P7 pups were exposed to either HT(35°C)+Xe(0%), NT+Xe(20%) or a combination of HT(35°C)+Xe(20%) starting with a 4 h delay after the insult. Brain damage was analyzed using relative hemispheric (ligated side/unligated side) brain tissue area loss after seven day survival. RESULTS: Immediate HT(32°C) (p = 0.042), but not HT(35°C) significantly reduced brain injury compared to NT(37°C). As previously shown, adding immediate Xe(50%) to HT(32°C) increased protection. Neither 4 h-delayed Xe(20%), nor Xe(50%) at 37°C significantly reduced brain injury (p>0.050). In addition, neither 4 h-delayed HT(35°C) alone, nor HT(35°C)+Xe(20%) reduced brain injury. We found no synergistic effect of the combined treatments in this experimental model. CONCLUSIONS: Combining two treatments that individually were ineffective (delayed HT(35°C) and delayed Xe(20%)) did not exert neuroprotection when combined, and therefore did not show a synergistic treatment effect.